Homebrew ¼ λ Magmount for 2m

Few parts are required to homebrew this magmount.

A 5¼” floppy disk drive rotor, with its boss removed, serves as the magnetic base. A thin plastic sticker, covering the exposed face of the magnet, prevents damage to the vehicle paint surface.

The enclosure is a suitably drilled Melamine or Bakelite cup on which the SO-239 is mounted.

2m ¼ λ Magmount details

RG-58/U or smaller coax is used. A ¼ λ counterpoise of stranded insulated hook-up wire is soldered to the braid of the coax. This is a must in case the rig is to be kept isolated from the body of the vehicle.

The enclosure is potted with epoxy to waterproof it and make it base-heavy. The same epoxy holds the assembly in position on the magnetic base.

The driven element is a ¼ λ length of 1.6mm brazing rod soldered to the PL-259 pin. The space between the PL-259 body and the driven element is filled with epoxy to prevent water ingress.

This magmount proved its usefulness on many occasions when access to the vehicle battery was denied and a separate battery had to be used.

Related post: Simple ¼ λ Ground Plane for 2m
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HT-powered CW Interface

A keyed piezo beeper, connected to the microphone jack of a HT, appeared feasible as a ready-made MCW generator.

It failed on 3 counts (high pitch, low volume and interrupted carrier) resulting in a very poor-quality signal.

Hence a bit of design effort was called for. The result is the following schematic.

HT-powered CW Interface - Schematic

It's a keyed audio oscillator, with a low part count, working off 4.5V - 1.5mA available at the microphone jack.

Oscillation is obtained using an AC128 (Germanium PNP AF transistor) and an audio transformer with a turns ratio of 1:5.

The 220K and 4.7K trimpots enable adjustment for the desired tone without motorboating.

In the event of the circuit failing to oscillate, connections to one of the transformer windings is reversed.

Connection to the microphone jack is through a shielded cable.

HT-powered CW Interface Board

The prototype was wired on a piece of perforated board with a microswitch serving as the PTT.

The unit was tested/adjusted while monitoring the signal with another rig. 100% modulation, with a clean note, was obtained.

Use of a homebrew electronic keyer precluded the need for a sidetone monitor.

It was an interesting weekend project using parts from the junk box.
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